confused about the zoom and variable aperture.

[thereforeiamx]

I've always been confused about the workings for the zoom for my Sony A100 kit lens [18-70mm]. When I zoom in or out, the aperture [or the shutter speed, depending on the mode] fluctuates. Why does it do this?

p.s. I would also appreciate it if any more experienced photographer would like to exchange PMs or screennames to help me out a tad

 

[Big Mike]

Every lens has a maximum aperture (lowest F number)...that's just a limitation of it's design. Many zoom lenses have a variable maximum...so when you zoom out, the max aperture gets smaller. (sort of)

The F-number is actually a ratio between the diameter of the actual aperture (hole in the lens) and the focal length. So when you zoom out, you are changing the ratio, which changes your maximum F-number.

The shutter speed fluctuates because it's tied to the aperture (in any of the auto modes).

Also, when you zoom, you may be changing what the camera's meter is reading...so instead of a small bright spot, you might have the whole image filled with the bright object...which would change the meter reading...which would change the shutter and or aperture values.

 

[Sideburns]

big mike basically just said exactly what I was gonna say...

If the focal length is longer, the relationship of the focal length to aperture opening changes, so your fstop number on your display will change.

 

[jwkwd]

It's like if you look through an empty paper towel roll. The diameter is the same the whole way, but if you cut it in half more light gets to your eye. Now, just how bizarre an explanation is that, but think about it.

 

[JIP]

It's like if you look through an empty paper towel roll. The diameter is the same the whole way, but if you cut it in half more light gets to your eye. Now, just how bizarre an explanation is that, but think about it.

Actually this is a good analogy the longer the tube gets (i.e. your lens) the smaller the end of the tube looks. So with your lens as you zoom out he aperture on the far end gets smaller and it lessens the light gathering capability of th lens.

 

[AUZambo]

It's like if you look through an empty paper towel roll. The diameter is the same the whole way, but if you cut it in half more light gets to your eye. Now, just how bizarre an explanation is that, but think about it.

Wow...you're a genius.

 

[BAB]

What has been said is correct, but know that it does not have to be that way for you. If the variability of the aperture is a problem there are zoom lenses, where the aperture remains constant throughout. The cost for these lenses are needless to say higher.

 

[Helen B]

Even for those lenses that have a variable maximum aperture the diameter of the entrance pupil very rarely, if ever, stays the same as you zoom. Just have a look at the ‘hole’ in your lens as you zoom. You can also calculate the diameter of the entrance pupil at the extremes of the zoom range. The kit lens is an 18-70 f/3.5 to f/5.6. That means that at 18 mm the entrance pupil is 5.1 mm diameter and at 70 mm the entrance pupil is 12.5 mm diameter. You should be able to see this when you look into the lens. The change is largely caused by the movement of the lens elements in front of the iris – the diameter of the iris itself does not need to change.

Best,
Helen

 

[DSLR noob]

What confuses me is that if you have a superzoom lens, like the Tamron 18-250 3.5-6.3 with such a difference, why doesn't the Viewfinder get darker when you zoom in all the way? Going from f/3.5 to f/6.3 should make a difference. it got brought up when I asked if a Canon camera (that can only autofocus with lenses maxing @f/5.6 or larger) can still autofocus with one of those puppies zoomed in making it a 6.3. Someone said yes and I wanted to know why. They replied somehting like "because the lens is a 3.5 at the wide end, the camera still sees through this 3.5 when autofocusing at any focal length, think about it, when you zoom does your viewfinder darken?" I still want to know if this is true....(if the same ammount of light seems to get through an opened up lens at all focal lengths) what makes variable aperture lenses..... well variate.

 

[Helen B]

The viewfinder does get darker - but it may not be noticeable. The meter can detect it. With split-image screens you may see darkening of the prisms.

Remember that halving the brightness of something (ie darkening by one stop) is not perceived as a halving - what we perceive as a halving of brightness is actually about a two-and-a-half stop reduction.

The Tamron lens you mentioned will actually collect more light from a part of the subject at 250 mm than it will at 18 mm, but the light it collects will be more spread out resulting in a lower brightness at the film/sensor plane.

18 mm f/3.5 = 5.1 mm dia entrance pupil.

250 mm f/6.3 = 40 mm dia entrance pupil.

Area ratio = 60x =(40/5.1)^2

The lens at 250 mm and f/6.3 lets in 60x as much light from a single point as at 18 mm and f/3.5.

But

The magnification ratio is 250/18 = 13.8 linear, or 192x by area.

Thus 60x the amount of light gets spread over 192x the area. That results in an image that is 60/192 as bright, ie 0.312 times, or about 1.7 stops. Not surprisingly, that is roughly equal to the f-number ratio squared (3.5/6.3)^2.

Does that help or hinder?

Best,
Helen

 

[DSLR noob]

That actually helps, but the jist of it is that the human eye percieves darkening in a different way than true light measurement.

 

[Helen B]

Yes. The famous 18% grey card was originally chosen because it is perceived as being half way between black and white, even though 18 is not half way between 0 and 100!

Best,
Helen